A biosensor is a sensor for measuring a substrate, by a biocatalytic reaction, namely by allowing a biocatalyst to react with a compound, a substrate thereof, which sensor has, as a transducer, a device capable of detecting a product resulting from the biocatalytic reaction, a decreased substrate or a chemical compound generated by a reaction with the product. Or, it means a sensor for measuring the substrate having also, as a transducer, a device capable of detecting physical signals such as a change in light and/or color, or fluorescence, resulting from the biocatalytic reaction. Examples of the biocatalyst include enzymes, organelles, cells and microorganisms.
That is, it can be said that a biosensor is a sensor which converts a biocatalytic reaction into a signal which an electronic device can detect, by using a biocatalyst as a molecular recognition element and by combining a signal thereof with a transducer such as an electrochemical device, optical device or heat detection device, and thereby is capable of analyzing a substrate recognized by the biocatalyst. One of the representative biosensors is an enzyme sensor using an enzyme as a biocatalyst. For instance, for the purpose of measuring glucose (grape sugar), a glucose sensor has been developed, on the basis of a concept, wherein an enzyme oxidizing glucose is immobilized on the surface of an electrode such as an oxygen electrode and a hydrogen peroxide electrode, and the amount of oxygen consumed by an oxidative reaction of glucose and the amount of hydrogen peroxide generated at the same time are electrochemically measured.
Among enzyme sensors widely used at present, a sensor using an oxidoreductase is mainly used. A major principle thereof is based on a method for measuring, with an ampere meter, electrons generated when a reduced substance generated by an enzyme reaction at an anode is re-oxidized by an electric potential externally applied; or a method for measuring by a difference in the electric potentials generated between an anode and a cathode when the generated electrons are reduced at the cathode.
Also, as a method used in a simple blood sugar diagnostic apparatus or the like, a method comprising colorizing a reduced substance generated by an enzyme reaction such as hydrogen peroxide or a reduced artificial electron acceptor in accordance with a conventional method, and determining the color by an optical sensor has been employed.
In addition, as an example of a special enzyme, an enzyme sensor employing a luciferase as an enzyme, which is an enzyme derived from a light-emitting organism such as a firefly, has been also reported, which enzyme sensor is characterized by detecting light generated by an enzyme reaction where a substrate for the luciferase reacts. Yet, as for this method, applications are limited to the cases where a luciferase can be used, such as the cases where the objective is limited to the detection of a substance which is an substrate for the luciferase, such as ATP, or the cases where there can be employed a principle that when an antibody reaction is detected, it can be indirectly detected via an optical signal by labeling the antibody with the luciferase.    Non-patent Literature 1: Katz et al., J. Am. Chem. Soc. 2001, 123, 10752-10753